Abstract
NaBH4 sodalites were obtained by two new modified methods of crystallization: (1) autothermal synthesis and (2) crystallization with crossover from gel to melt flow in NaOH flux. Syntheses results were presented according to XRD, SEM and FTIR. Besides important features of both synthesis procedures product properties like crystal size and morphology were investigated. Spherical agglomerates of microcrystalline sodalite of composition Na7[AlSiO4]6BH4(H2O)2 were already observed after 4 h without any external heating by the autothermal procedure. Sodalites of the same average composition but in form of agglomerated nanoparticles are crystallized after very short times (2 h 30’) by the crossover reaction from gel to melt flow. Hydrogen release by heating was further studied for two selected samples with comparable composition from each synthesis procedure. Total hydrogen release by hydrolysis reaction with the internal cage water was found during heating of the autothermal product in synthetic air up to 550°C. In contrast hydrogen release from the nanocrystalline sample of crossover synthesis was not completed when heated under the same conditions. These differences were discussed in terms of crystal size and an earlier loss of the internal water from the nanocrystals of the crossover synthesis
Highlights
Sodium-tetrahydroborate (NaBH4) is a representative material for hydrogen storage and contains 10.6 wt% H2 [1] but is a moisture sensitive compound
For a statement of the mechanism of this procedure one can consider the whole reaction as a crossover synthesis in gel-like solution followed by crystal growth in a melt as a one-pot process
As a result of the rapidly rising temperature within the strong exothermic reaction system this process is overlapped by NaBH4-sodalite crystallization under inclusion of BH4 anions according to Equation (3): 6 Na (AlO2 )(SiO2 ) + 2 NaBH4 → Na8 [AlSiO4 ]6 (BH4 )2 (3)
Summary
Sodium-tetrahydroborate (NaBH4) is a representative material for hydrogen storage and contains 10.6 wt% H2 [1] but is a moisture sensitive compound. The autothermal synthesis (1), first described for hydrosodalites in [10] and the low temperature crossover process from gel-like aqueous solution to melt flow (2), first introduced for halide sodalite in [11]. Both new enhanced methods will be described in separate sections. The crossover reaction from aqueous gel solution to low temperature melt flow is demonstrated here as a further method for tailored synthesis of NaBH4sodalite. For a statement of the mechanism of this procedure one can consider the whole reaction as a crossover synthesis in (aqueous) gel-like solution followed by crystal growth in a melt as a one-pot process. The present work demonstrates this new preparation technique as a case study of insertion of a modified method for tailored synthesis in the NaBH4-sodalite system for the first time
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